1. Field of the Invention
The invention relates to a method for detecting the position of a moving material web, as well as to a device for carrying out the method.
2. The Prior Art
A photoelectric sensor system is described in German Patent No. DE 34 23 308 C2, in which a light source is disposed opposite a plurality of independently acting receiver components. The receiver components are formed by semiconductor photocells, which are optically coupled to the surfaces of plastic panels. Each of the receiver components detects a defined zone of the material web, and the photocell sends out a signal that is proportional to the coverage by the material web. The position of the edge of the material web is determined by comparing the signals emitted by the photocells with a threshold value. This method has the drawback that the resolution achievable with such a sensor system substantially corresponds with the width of the plastic panels. Therefore, to obtain a high resolution combined with a large measuring range, a very large number of photocells is required, which makes such a sensor system expensive. Furthermore, a very large number of photocells must be interrogated in each measuring cycle, which leads to intolerably long measuring times.
A device for detecting the position of the edge of a moving material web is described in German Patent No. DE 42 09 546 C2, which shows a receiver component disposed opposite each of a plurality of separately controllable light sources. The receiver components emit a signal that is proportional to the coverage by the material web, and which is compared with a threshold value. The result of this comparison is supplied to a microcomputer that controls the light sources and the receiver components depending on the result of the comparison according to a successive approximation, to determine the position of the edge of the material web. To carry out this successive approximation, it is necessary to store a number of comparative values of the receiver components with the threshold value in a memory unit of the microcomputer.
It is therefore an object of the invention to provide a method of the type specified above that achieves high resolution in a short measuring time period.
It is another object of the invention to provide a device for implementing this method at a favorable cost.
These and other objects of the invention are accomplished by a method for detecting the position of an edge of a moving material web, in which the material web is scanned by a plurality of sensors distributed transversely to the moving direction of the web. In this process, each sensor generates a signal that depends on the amount that the web covers the sensor. The signal generated by the sensor is preferably proportional to the coverage by the material web. In this way, each sensor supplies a signal that depends on the position of the edge of the material web, and the individual sensors are arranged in different zones in the measuring direction.
The material web is preferably scanned without the web coming into contact with the sensors, so that even webs consisting of a sensitive material will not be damaged by the sensors. Furthermore, the sensors themselves are prevented from interfering with the detection of the edge, as compared to a mechanical edge sensor that is pressed against the edge of the material web by spring force. With the edge of the material web being in about the mean position, the inner sensors viewed from the center of the web are completely covered by the material web, and emit a constant idle signal. The outer sensors are not covered by the material web at all, and generate a maximum signal. The sensors located in the zone around the edge of the material web supply signals that are between the idle signal and the maximum signal.
Therefore, the function of the signals generated by the sensors from the sensor locations have two constant ranges of different signal intensities when the edge is in about the mean position, with a constant transition range between the two ranges. If the edge of the material web is in an extreme position, i.e., near the innermost or outermost sensor, one of the constant ranges may also be missing, so that the determination of the edge via threshold values becomes very inaccurate. It is therefore proposed according to the invention that the position of the edge be determined based on the turning point of the function of the signals generated by the sensors from the sensor locations. This turning point can also be determined when the edge of the web is in an extreme position even if the sensor signals can no longer be determined with complete coverage or complete release of a sensor by the material web.
The individual sensors are preferably distributed equidistantly transversely to the direction of movement of the material web, so that the sensor location can be determined in a particularly simple way by numbering the sensors. In this way, the detection of the turning point can also be determined based on the function of the sensor signals by th e sensor number. The result of the search for the turning point is multiplied by the mutual spacing of the sensors. The turning point can also be generally determined based on a function of the sensor signals of values that are proportional to the sensor sites. In order to obtain a correctly scaled web edge position, the detected turning point has to be subsequently multiplied by the proportionality factor. In those cases in which only a signal proportional to the edge position is required, which is the case with web guiding devices, it is possible to eliminate a corresponding correction of the determined reversing point.
The sensor integrates its signal from a predetermined area, so that deposits with small-sized surface areas resting on the surface of the sensor have only a minor influence on the signal generated by the sensor. This is important in connection with material webs that lose a lot of fluffy matter in the form of lint, because the lint originating from the material web may directly deposit on the surface of the sensor. If the sensor had an effective width below about 1 mm, each individual lint fragment could cause complete coverage of a sensor, which would be erroneously interpreted as an edge of the material web.
Due to the design of the sensor with a large effective surface area, lint deposits may only lead to a uniform dampening of the signals of all sensors, which has no influence on the position of the turning point. Increased insensitivity to dirt of the edge-sensing system is therefore achieved. Since the turning point can be determined with substantially greater accuracy than the spacing of the individual sensors, this measure will not become a burden on the accuracy of the detection of the edge.
The methods for detecting the coverage of the material web work without contact with the web, and consequently do not interfere with the movement of the web. It is also possible to safely scan webs of particularly sensitive materials without the risk of damaging the material web, or the risk of any falsification of the measured values. The coverage by the material web is optically detected either by the reflection method or based on the light barrier principle.
The material web is illuminated by at least one light source, with light-sensitive receivers either on the side opposite the material web, or next to the light source. Alternatively, the coverage of the material web could be scanned pneumatically by air jets. This requires pressure-sensitive sensors. With acoustic scanning of the material web, sound waves are aimed at the material web, which are then detected with the help of microphones.
The function of the signals originating from the location of the sensor are differentiated two times numerically and a zero coefficient is determined based on the second derivative. The zero coefficient of the second derivative conforming to the edge position is located in the area of the greatest ascent, thus at a maximum value of the first derivative. Based on these conditions, it is possible to exactly determine the position of the edge wit h low computation expenditure.
Alternatively, it is possible to determine the turning point through simple numerical differentiation and numerical searching of the maximum of the first derivative. A second differentiation is thus saved. However, the search for the maximum of the first derivative requires greater numerical expenditure. However, this method offers the advantage that the condition of searching for the absolute maximum supplies the position of the edge of the web without any further secondary condition.
Alternatively it is proposed to approximate a fit function to the function of the signals generated by the sensors from the sensor locations, so that the turning point is directly calculated based on the fit parameters. The fit function is preferably approximated to the signals of the sensors by minimizing the squares of the deviations of the signals from the corresponding functional values of the fit function. The turning point generally can be computed analytically by two-times differentiation and putting the fit function to zero, so that depending on the fit function, it is possible to state a simple formula for calculating the turning point based on the fit parameters.
The fit function is preferably a polynomial whose coefficients are the fit parameters. Based on this polynomial, it is possible to analytically calculate the second derivative in a very simple way, so that the turning point can be directly determined based on the coefficients of the polynomial.
The signals of the individual sensors can be divided by the signal of a reference sensor. Measuring errors originating from a change in the illumination intensity of the emitter can be eliminated through this measure. If the signals of the sensor and the reference sensor are detected close to time, short-time variations of the emitter can be compensated as well. A sensor is preferably alternately interrogated by the reference sensor in order to compensate emitter variations in as optimal a way as possible.
The device according to the invention is successful for implementing the method as defined by the invention. The device has a plurality of sensors, which each generate a signal depending on the coverage by the web. The sensors are preferably set up next to each other in a row extending transversely to the direction of movement of the web. As an alternative, the sensors also can be arranged offset relative to one another in a zigzag configuration. The individual sensors must be located differently spaced from an edge of the web, imagining the web edge as a stationary edge. Each sensor thus reacts to changes occurring in the edge of the web within a defined range of the width. If t he sensor is completely covered by the web, it only generates an idle signal. If the sensor is not covered by the web at all, it generates a maximally active signal.
The individual sensors a reconnected to an A/D-converter via switching means. The signals of the individual sensors a re queried successively via the switching means and converted into a digital value. The switching means are actively connected to storage means, where the digitized values can be stored. A turning-point detector, which determines a turning point based on the function of the digitized values from the sensor locations, can access the storage means. The digital value computed by the turning-point detector is issued to a digital output and is thus available to further devices such as a web edge regulating means or display instruments.
So that the desired integrating effect of the individual sensors is achieved with the simplest of means, the sensor is designed in the form of a photosensor with an associated panel made of light-collecting plastic. The plastic panel contains a fluorescent substance that transforms the light coming from the light source into a light with long waves, which preferably propagates in the plane of the panel. The photodetector is coupled to the face side of the panel, so that it receives a very strong light signal and consequently a light signal that is insensitive to interference. Due to the appearance of total reflections of the light on the surfaces of the panels, the light is evenly distributed over the entire panel, so that the sensor supplies a signal that is exactly proportional to the coverage provided by the material web. It is not important whether the light penetrates the plastic panel with uniform distribution or only in a certain site.
The web edge detection is impaired by light rays directed inclined relative to the plane of the material web, especially if a large spacing exists between the light source and the sensors. There is preferably an element located between the light source and the sensors that parallelizes the light rays. This element is in the simplest case a body with a multitude of surfaces aligned perpendicular to the material web. For example, the element could be formed by tubular pieces, walls extending parallel with each other, or a honeycomb structure. As an alternative, the element could be formed by a row of lenses or a bundle of glass or synthetic fibers. It is important only that the light rays, viewed in the direction of detection of the device, are aligned parallel with one another. On the other hand, viewed in the direction of movement of the web, different directions of radiation of the light only play a subordinated role.
The switching means for interrogating the sensors in a time-shifted manner is preferably an analog multiplexer. An analog multiplexer is available in the form of an integrated circuit, which is consequently insensitive to chemicals and elevated temperatures that occur in the processing of textile material webs.
So that the signals generated by the sensors will not be falsified by the On-resistance of the analog multiplexer, which is not negligible, the signals are transmitted not as voltage but rather as current values. In order to permit subsequent analog-to-digital conversion, a current/voltage converter is arranged downstream of the analog multiplexer, and generates a voltage proportional to the sensor current.
If the device is employed in a very hot environment, it must be capable of handling thermal expansion. Since the individual components of the device are made of different materials, the sensitive motherboardsxe2x80x94on which the sensor electronics is mountedxe2x80x94must be designed so that they are somewhat expandable. The motherboards supporting the sensors are therefore designed at least partially flexible. It is conceivable to build up all of the sensors on at least one motherboard that is flexible over its entire length. As an alternative, the sensors can be accommodated in the form of modules set up on hard motherboards, with a flexible connection via cables between each of the individual modules.
The circuitry of the turning point detector is costly, so that the detector is preferably designed in the form of a controller. The required computing steps can be accomplished by a suitably devised operating program, so that the circuitry expenditure is kept within justifiable limits. The required A/D-converter is preferably designed as a component of the controller as well, which further reduces the circuitry expenditure.
Finally, there is at least one D/A-converter which can be designed as a component of the controller as well downstream of the turning point detector. The digital-to-analog converter generates an analog signal that is proportional to the turning point and thus to the position of the edge. The analog signal can be directly supplied to a web edge regulator.